This invention relates to assessing the dielectric capability of a vacuum device, and to conditioning such devices to improve the dielectric properties thereof. More particularly, it relates to using the radiation generated by field emission current as a means for determining the amount of that current, and to using the radiation to determine the field enhancement factor associated with the dielectric state of the gap between the cathode and the anode of the vacuum device.
It is often desirable to know the dielectric strength of the gap between the cathode and the anode in a vacuum device. Perhaps the simplest way of testing the dielectric strength of such a gap is to apply sufficient voltage between the cathode and the anode to cause breakdown. However, this simple procedure is not a reliable determination of the dielectric strength, because breakdown voltage in a given trial is highly variable and many trials are required to obtain a reasonably accurate measurement of the true dielectric strength. Moreover, this procedure, in and of itself, may cause physical damage to the electrode surfaces, and thereby alter the results obtained for subsequent measurements of the breakdown voltage. A theoretically more accurate method of determining the dielectric strength is to measure the field emission current between the cathode and the anode as a function of applied voltage, where the voltage approaches, but does not exceed, the expected breakdown voltage of the gap. The dependence of emission current upon the voltage between electrodes is well known and described by field emission theory. Furthermore, this emission current can be related to the dielectric strength of the gap, as described hereinbelow.
In many circumstances, breakdown in a vacuum gap has been found to occur at a relatively constant electric field strength. This field, which is often referred to as the critical field, E.sub.c, tends to have a value which is a constant characteristic of the electrode metal employed, and typically has values lying in the range of between 10.sup.7 and 10.sup.8 v/cm. The magnitude of this electric field suggests that vacuum gaps are capable of having extremely high dielectric strength, such as 10 million volts across a one centimeter gap. In practice, however, this capability is seldom realized. One reason for this reduced capability is that the average electric field impressed across the gap between electrodes is almost always enhanced by a factor of between one and three by the shape of the electrodes, since electrodes used in practice seldom are designed to have perfectly flat facing surfaces. A much more important reason is that the microscopic features of the electrode surface, even those too small to see under optical microscopy, can enhance the electric field by a factor of 100 or more. Thus, while the critical field for typical electrode metals is as high as 10.sup.8 v/cm, the strength of a vacuum gap in practical devices is usually reduced to the order of 10.sup.5 v/cm. The field enhancement factor which accounts for this reduction in electric field strength is usually called .beta. and includes the combined effects of electrode design shape and microscopic surface structure. The field enhancement factor for a particular vacuum gap is influenced by such parameters as electrode material, surface hardness, cleanliness of the surface, and electrode conditioning. The relationship among the critical field, E.sub.c, the breakdown voltage, V.sub.B, the gap length, d, and the field enhancement factor, .beta., may be written as EQU V.sub.B =(E.sub.c d)/.beta..
Since the critical field and the gap length for a particular vacuum device are constants, it can be seen from this equation that .beta. is small for gaps having a high breakdown voltage, while .beta. is generally large for gaps which break down at relatively low voltage. Thus, .beta. is an indicator of the general dielectric state of the vacuum gap.
Under certain conditions, the factor .beta. can be determined by measuring the emission current as described herein, that results from applying a voltage, at a level somewhat less than that required to produce breakdown, across the vacuum gap between the cathode and the anode. This emission current increases rapidly with increasing voltage, up to the breakdown voltage for the gap. As discussed at pages 24-35 of the book edited by J. M. Lafferty, Vacuum Arc--Theory and Application, John Wiley and Sons, New York, 1980, the dependence of this current upon voltage is described by the theory of Fowler and Nordheim as EQU I=K.sub.1 V.sup.2 EXP (-K.sub.2 /V)
where K.sub.1 and K.sub.2 are approximately constant. From the theory discussed therein, it can be seen that if the logarithm of the quantity resulting from dividing the emission current by the square of the applied voltage is plotted as a function of the reciprocal of the applied voltage, the result is a straight line having a negative slope. The numerical value of this slope, disregarding the negative sign, can be directly related to .beta.. Therefore, the emission current can be used to determine the value of .beta., which, in turn, can be used to assess the dielectric state of the vacuum gap.
However, field emission currents generally are small, typically being in the range of microamperes or smaller. For high voltage devices, leakage currents and corona currents can both easily exceed the emission currents to be measured. Also, unless the power supply used to apply voltage to the device is extremely well regulated and ripple free, displacement currents can be factors of ten larger than the emission current. Thus, measuring the emission current for a high voltage device can be very difficult. Furthermore, for a vacuum device, in which the cathode and the anode are enclosed in a vacuum chamber, measuring the emission current is very cumbersome because the gap between the cathode and the anode is not easily accessible. Finally, even if these difficulties can be overcome, there is a high risk of damaging sensitive current measuring equipment if breakdown should occur in the vacuum gap while the emission current is being measured. The present inventor has found that these difficulties can be avoided by using the radiation which is generated by electrons from the field emission current striking the anode as an analog for the emission current, rather than measuring the emission current directly.
One application in which it is useful to be able to determine the dielectric strength of the gap in a high voltage device is in the process of "aging" or "conditioning" the device. When a newly manufactured high voltage device is first subjected to voltage, the ability of that device to withstand high voltage is relatively poor. The dielectric properties of the device can be improved by a program of aging or conditioning which involves a series of steps of exposure to increasing levels and time periods of high voltage. Gap breakdown voltage capability can be improved by such a process by a factor of two or more. For this application, being able to determine the dielectric state of the gap allows optimization of the conditioning program, by facilitating the joint determination of which voltages and time periods are most effective and when to stop the process.
Accordingly, it is an object of the present invention to provide a method for indirectly determining the field emission current between the cathode and the anode of a high voltage vacuum device, a method which is not influenced by leakage, corona, or displacement effects.
It is also an object of the present invention to provide a method for determining the field emission current which does not require access to the vacuum gap.
It is a further object of the present invention to provide a method for determining the field enhancement factor, .beta., associated with the dielectric state of a high voltage vacuum device, without directly measuring the emission current between the cathode and the anode of such a device.
It is still another object of the present invention to provide a method for conditioning high voltage vacuum devices to improve the dielectric properties thereof, without breaking down the vacuum gap in such devices and without directly measuring the emission current between the cathode and the anode of such devices.